Electronic voltmeter with calibrated resistors



Filed Feb. 27, 1946 1952 M. FOUQUART 2,

ELECTRONIC VOLTMETER WITH CALIBRATED RESISTORS 2 SHEETS-SHEET 1 INVENTOR FL-FouQuART Jan. 1, 1952 M. FOUQUART ELECTRONIC VOLTMETER WITH CALIBRATED RESISTORS Filed Feb. 27, 1946 2 SHEETSSHEET 2' XNVENTOW M FOuQ. U AR T Patented Jan. 1, 1952 ELECTRONIC VOLTMETER WITH CALIBRATED nEsisTons Maurice Fouquart, Paris, France Application February 27, 1946, Serial No. 650,665

In France August 13, 1943 The present invention relates to a vacuum tube voltmeter adapted for use "for all electrical and radio-electrical purposes, the apparatus being based on the fact that the grid of a tube of the type commonly known in the radio art by the name of magic eye? (or tuning indicator tube) will consume substantially no current when said grid is brought to'a potential which'is slightly negative with respect to the cathode, say to. the extentofabout 3 volts for'the usual types of construction. It follows that it may be'used for measurement purposes without any voltage con- ;sumption and without the necessity of resorting, as in the prior art; to measuri'ng devices involving mechanical deflection of say, a movable coil or the like.

The arrangement of this voltmeter will be described with reference to the diagrams shown in the accompanying drawing which are given merely by way of examples. In the drawing: r

Fig. l is a diagram showingv the principle the measuring procedure used.

Fig. 2 is a diagram showing modifications made in the device used in Fig. 1' in order to adapt it portable voltage '7 Claims. (01. 171-95) two luminous sectors;

The switch is then placed on contact 2 so that paratus (ground) by means of contact I or to the +V pole of the battery through the contact 2.

- Let-it first be assumed that the slider of the is then adjusted once and for all so that the grid bias thus produced is suificient both-to effectsup- 'pression of the grid current and to position'the image on the tube at a point that can'be readily indexedorlocated, for example as the junction of the voltage to be measured is applied to the grid.

The image inthe tube will vary immediately. In order to reproduce the initial image without disturbing the adjustment of R01, the resistance of the rheostat Rc must be altered in such fashion as to produce between 01 and the ground, a voltage that is equal to the voltage V that is to be measured. 7 i

In lieu of measuring this voltage with the aid of an ordi'nary voltmeter,'it can be determined in accordance with the present invention by passing a current of fixed intensity through the rheostat Re. In order to produce this unvarying current, use is made of the voltageHT which is regulated by the neon tubes N, and the operating In Fig. 1, the voltage V which is to be measured is that prevailing between the terminals of a battery B.

1 +HT and HT designate two terminals con-' nected to a source of high tension and between which are interposed the neon tubes N which regulate this voltage. The HT terminal is furthermore connected to the body of the'framewo rk (grounded) as is also the pole of the battery B. In practice the body of the framework is preferably grounded to the earth in the majority of cases.

i c, g, a designate the cathode, grid, and anode -of an ordinary vacuum tube of the so-called tuning indicatorftype. The anode a is connected to the i-HT terminal, through intermediary of a rhecstatRa. The cathode c is connected to the body of the apparatus (grounded) through intermediary of two .rheostats in series, Rcland R0.

Furthermore, a switch Cpg makes itpossible to connect the grid 9 either to the'body 0f the apshafts of the rheostats Rd and Re are so connect- 1 ed that the sumof the resistances'of these rheo- 3 1 stats is always constant.

It is apparent that if the impedance of the tube between a and c aswell as the impedance of RC1 remains consta'ntythe fact that Rc-l-Ra is rendered constant'irrespective of the variations of Re will result in rendering the total impedance between +HT and --HT constant and consequently serve,unde'r the influence of the constant voltage prevailing betweenl+HT and HT to produce a current of constant intensity in Re, RC1 Ra.

The position which the slider of I the rheostat Rc mustoccupy in order to reproduce the initial image at the electronic tube will therefore be a function determined by the. voltage V, so that if this rheostat Re is calibrated (obviously for a fixed value of R0 1) it will serve to measure the voltage V; this calibration should be preliminarily effected with: the aid of calibrated voltage sources or determined by calculation and it may be marked off in volts, I

The arrangement thus constituted, may, in practice, 7 be. embodied in various ways, depending on the applicationscontemplated and the nature of the voltage sources utilized.

For example, in case the negative poleof-the voltage" to be measured cannot be connected to "rigidly coupled together. ments of the usual radio switch of the 'B-circuit,

the body of the apparatus (grounded), for example because of the necessity of keeping the insulation perfect, the device described above may be modified as shown in Fig. 2:

In this case, three switches Cob, Cgb and Cab may be used to connect the cathode, the grid and the anode of the electronic tube to three respective groups of "terminals 41,, 2; 3. .These switches are so interconnected as -simul'taneously to close the three contacts I or the three contacts 2 or the three contacts 3. and terminals of the voltage. source HT, regulated by the neon tubes N, is connected a potentiometer Pb, the slider of which is connected to the terminal of the sourcezo'f voltage V that is to be measured and .to the .terminal .I of the switch Cgb.

The cathode is connected to the body of the apparatus (grounded) through intermediary :of the rheostat Rc1, the switch Ccb and the rheostat .Rcb. Moreover, the grid :may :be connected by means of switch Cgb either to the terminal of the voltage tOsbeflIBfiBllH-Zd or to thez-rheostat :R'c1 ,:cr 'to the terminal :of the voltage that is to be measured. The rheostats Rob and .Rab Rare coupled together mechanically -1ike Ba and Roof Fig. '1.

When the=three switches of.Fig.'2 are in posi- ---tion .2, adjustment of R01 :is .made once for .all. =.'-Ihe'-image of -the tube is -.positioned, as already recited, at an easily identified point. Position i is then resorted .to. It will then berperceived that in (order to regain the original image, it is .necessary :to adjust the potentiometer :l'b rat a 3 or the measuring position.

It :is apparent that in (order :always .-to :regain the .same .image, Rob/Rab must be manipulated .in fashion that: Measured -voltage=maximum ..measurable voltage-the woltage read off at ..Rcb/R'ab.

In order sto simplity matters, the ,pointof comparison (position 2) may be taken, :not at 1,611),

. (or :Ccb) :after .having brought :the resistance :of ..Rcb to a :maximum .and :nullified the gresistanoe of Rab, but simply taken at the .rheostat .Pb,

which will eliminate the use of two switches.

This simplified arrangement :is utilized :in the embodiment shown in;Fig.

This .figure shows the two anodes .P1, P2 and the targetrziof the magic:eye (tuningindicator tub).

MAand. MB indicate the terminals .to whichthe voltage X to be measured is applied.

.RA, RB, RC, RD,.RJF, .RK, RL, .RM, RN, .RO, RP,.RP1,. RP2 designate resistances of whichsome are fixed and .othersadjustable.

CDA, .CDB, .CDD .are the resistance switches, arrangedin series, the contacting members of CDA andCDC (1U resistances .rinseries,

for example) -being coupled together in such fashion .thatthe sum of the resistances utilized in each pair of rheosta'ts'is constant. The same applies as to (DB which. iscoupled .with 'CDD (9 resistances R, ten times stronger, in series, .for

example, to obtain a sum total of 1001').

CLI, CK, CF are three two-position switches These are three ele- 2 position type, for example.

Twopositions, for example, correspond to the 4 two reading scales desired. (10 and 100 volts or 50 and 500 volts.)

' The number of scales used is obviously not limited to two.

Likewise the contacting members CR, CS, CT, CV, CP, CH are rigidly coupled together. CL is always used for passing from the index or identifying point to the point ofoomparison.

C2 is .used for imparting a, multiplying effect 10 to the measuring apparatus by causing it to op- Between the 1erate as a potentiometer (non-infinite resistance in this single case).

1N designates the neon tubes which serve for regulating the voltage of the supply source.

.15 .Two potentiometers P10 and P100 make it possible to bring the terminals of the switch CF to two -predetermined potentials corresponding to the two scales, for example, 10 and 100 volts.

Another potentiometer PM is used only when the magic eye tube is replaced by another,

,for the purpose of compensating 101' any variationin the total regulated voltage. V 7

One of .the .ieatures .of .the invention consists,

.as recited above, in automatically .indicating .in

525 theanode circuit the valueof the resistance used .for decreasing thetcathode .circuit 0.

vIf .UG .is .the total potential difference between 0am! HT or the general overall regulated volt- .age,

: UC the difference -.in potential between .the

.cathode and Q,

, .UL the difierence .in .potential between the .cathodeand the .platesxof the magic eye .tube. the difference .in potential between the plates and Hit, it .follows, from .the original .switch connections used, that, if (by definition) =UG=rIc+UL+Ua we would likewise obtain, irrespective :of the var iations assigned itO .UC, itheyalue UC+UP=constant,

-whence UL: constant Furthermore, if, :during the various measurements made (necessary to facilitatemeading) the difference :injpotential between the grid and the ground is constant and slightly-negative, :a :per-

,fectly :constant current will flow through .the

resistances.

Consequently, with theaid unknown-calibrated and 'unvaryingiresistances :the operator can produce a difierence in potentialothis own choosing which-can be read off at the resistance switches,

zeither'theiswitch CDA'.WhiQh is'vmechanically coupled with CDC and calibrated for examplezfrom .0 to 10 :volts, or at the switch .CDB which .is :me-

.chanically coupled to CD13 and calibrated for .examp'letfrom 10 :to *.volts, in :10 volt increments.

.It is merely necessary .to use "the compensaition potentiometer JElVl'. if or regulating .the :current dzoaknown value so as to be 2313161170 determine the difference in potential over the known potential since it-varies as .adirect function of this value at UC.

In a test apparatus, in order-to obtain the multiplying effect hereinafter discussed, I may be chosen as equal to 4 ma. (milliamperes), r=25 ohms, R=250 ohms; so that .1 volt would be obtained .for each variation ;at .CDAor .1 volt .as a total and 1 volt for each variation ,at .CDB or 9 volts as a total for CDB.

If the output in the measuring resistances is "increased by means of switches .CL1 and CK, and" -the zero of the measuring switches.

the power consumption of the system is brought to The cathode resistance RA yields a cathode voltage that is slightly positive with respect to The grid is therefore slightly negative with respect to the cathode, which'is essential to'avoid any power I output.

This resistance moreover makes it possible to locate the permanent operating point of the magic eye tube at a position where the shadow'a'ngle is greatest (in both directions) for agiven low potential difference. p q

If, in order best to fulfill the later condition,

'itis desired to vary this point of operation, a compensation system may be used similar to that shownior the measurement switches CDA/CDC and CDB/CDD per se.

If for example it is desired to have three shadow angle regulatin positions, a variable resistance 7 system is connected in at am in such fashion that for calibration purposes,:-a millivoltmeter having a low, potential difference between its terminals,

which may furthermore be compensated in practice by means of an equivalent resistance.

Furthermore, the two terminals V1 and V2 make it possible to verify results by means of any voltmeter. It is .to be noted that this voltmeter involves but a' small power consumption compared to the total consumption since the simultaneous coincidence of the measurements taken at the voltmeter and at the switches is perfect.

At allevents the agreement with the voltage tobe measured is exact if the switches are not taken into account.

- The potentiometer PM, in series with the resistance'RB makes it possible to adjust the consumption of the circuit as a whole to the first comparison intensity.

It is obviously possible to measure negative voltages by placing the negative point at MB, but this method of procedure is unsuited for the use for which an electronic voltmeter is intended, that is, for measuring small outputvoltages (E. M. F. at high internal resistances); if the point MA, connected only to the grid of a tube shows perfect insulation, this will not be so at MB where the slightest fault is not attributable to any direct connection with the secondary of the supply transformer.

The solution described above be used.

if l iegeneral diagram (Fig. 3) shows ,how the problem of measuring negative voltages is solved.

would therefore For this purpose the switches CH, CP, CS, CT,1

CV, and CR are operated so as to set them on the negative instead of the positive'contacts;

' The point MB is then brought, with respect to the ground, to a positive potential equal to the maximum voltage of the scale employed'( volts and 100 volts, for example); the potentiometers P ocand P10 make it possible to adjust these voltages to suitable values. It will be noted that these various potentiometers need not be manipulated during each operation, but are to be manipula'ted only during starting and for verifyingresuits from time to time.

If calibrated resistances are used in the apparatus, the 10 and volt potentials may remain fixed, although 'it is preferable to be able to readjust these basic voltages. The values 10 and 100 however are merely arbitrarily chosen by way of example. v I

The switch OK and the resistance RK serve to provide the voltage necessary for the measurements, either 100 volts or 10 volts, alternatively, in the case under consideration.

The neon tubes N are in practice sufiiciently constant for use inregulating the required voltage but since they differ between each other so far as the regulated voltage is concerned, the +100 volts is not taken off between the first and second tube. I In principle, the fixed comparison voltages of 10 and 100 volts are adjusted so as to effect closingof'themagic eye to an extent equal to that commonly used in eifecting positive measurements.

A novel method for reversing the terminals CR, CS, CT, CV of the rheostats associated with each switch makes it possibleto preserve the same reading on the instrument board.

In the absence of this special system, the switches must be doubly calibrated, as follows:

' For positive voltages: o, 1, 2, 3, 4, etc. For negative voltages: 10, 9, 8, 7, 6, etc.

RL and the electrolytic condenser CFT constitute a filter unit with CFN. These condensers may be disconnected readily when taking positive measurements; an inductance may be inserted at XT.

The resistances RM and R0 alongwith the capacity of the conductor MA constitute a filter (which is preferably variable), so as to produce 'a given intensity; the same measuring voltage would always be available, the or volts of the overall voltage only being indicated on the tube.

However, fresh regulation is necessary whenever a tube is changed (that is, the magic eye tube, and especially the regulating tubes) of the apparatus, a known test such as 10 or 100 volts, for example, which can be verified with the aid of 'a line voltmeter of any suitable type.

The possibilities for using this apparatusmay moreover be extended in the following manner: In order to reduce to a minimum the input capacity without however rendering the grid sensitive to parasitic oscillations (effect of the hand on the steatite prod, or alternating voltage resulting from capacity effects) the principal circuit terminals of the (magic eye tube may be connected by means of a-switch to a socket disposed on a support. By means' of a suitable prod and cord the magic eye tube OM can be led off to amovablesupport 1i (Fig.4 shaped like a pistol, the visible portion of the magic eye" tube being turned towards the operator. The support-for the magic eye tube (support and :sleeve preferably made of steatite) ,is providedwitha rod2 (shielded) in lieu of a grid, and adapted to efiect .contact with a minimum of loss due either to capacity eiTects or resistance contact eiiects.

' sGontact isefiected through the flexible blade 3 .(silver contact and steatite insulation) operated by re. ftrig cr 4 of the portable apparatus and serves to connectthe-grid of the magic eye tube either-with the contact prod 5 or with the point ofzcomparison such-as CL (Fig. 3), the grid being then connected directly to the circuit that is to be "measured without any series resistance or para lel capacity, the point MB of Fig. 3 being also connected in. The absence of "all parasitic oscillations'therelore :makes it possible not only to measure the continuous component of a continuous modulated voltage but also to evaluate the modulated component of such voltage.

.In the absence of such a system, comprising :apor-table magic eye tube, the grid measuring icircuit-would have to be very thoroughly insulated and if possible, shielded (i. .e. of the low- -loss antiparasitic antenna type).

It should however he noted that when taking negative measurements the switch CL does not permit making the same comparison as .in the case of positive measurements.

In order not to encumber the circuit of Fig. 3 with another switch, it need merely be brought itoizero with theaid of scales arranged in tens so as to-obtain the-comparisonpoint.

An interesting application of the apparatus resides inits use for measuring the negative oscil- -lation voltage at the terminals of the oscillation resistance of a vacuum tube, a-f6-A-8 tube for example, in-which case the point MB is connected .to the-{cathode :and the point MA directly connected to the oscillator grid -(the resistance of 8 megohms being disposed at the end of the nipper-equipped prod). It is possible to observe \(i. B. -see the variation at the tuning indicator tube (magic eye) in proportion asithe oscillation condenser ,of theradio set undergoing examination isturned.

- This use .is especially interesting for verifying oscillation factors in radio receivers of the frequency changer type (i. ,e. by short-circuiting the variable condenser of the oscillator circuit, the residual negative voltage can be measured) and for adjusting and checking oscillator coils, -especially those intended for short wave work.

0L1, .CF, GK for example i are three i of the circuits of a switch of the single bar, l-contact, 3- position type the i0 volt, 100 volt scale is used; (the 4th contact cuts the primary of :the, feed transformer) Furthermorait is apparent that in View of the .ZBlO consumption ,of the apparatus the presence of a resistanceat RM will not=produce .any;-ap-

,mfeci-able variation in thereading. 'On theconi Voltage to be ineasured-voltagereadx R M +13 N irnerr sisanees RM an .R onstit te he-ele- ;ments..of a-filtemdanted teammat -alternatin components and esp c all ific mnonentstil z In-practice, if high values beassigned to the resistances RM and RF, :itimay be suflic ent, in

order to eliminate :all disturbances, to add asingle capacity at Co, to use a shield fmagic eye tube, and to use the capacity of the circuit the connects MA to the grid.

"The magic eye tube has the following advantageousproperties:

1. Very great sensitivity in one of the shadow sectors.

2. Perfect luminosity, for examplecapable of giving luminous screen images making it possible :to observe very slight variations in the shadow sector. I

3. The support of the tube is pi steatiteor of the Lok l 1 s as ftyp Incaseswhere alow overall regulating intensity is util zed th h a ing o t e be a ct h output, in which case-it may be preferable to insert an iron-hydrogen intensity regulating tube in the primary of the feed transformer.

It is obvious that the invention is not limited to'thecircuits described, which are merely shown by way of exampl without any intention of limiting the scope.

It is possible with the aid of the same apparatus to measure high resistances as bYconnecting the same with a series resistance ofknown value and measuring the voltage .drop between the terminals of one of these resistances; the ratio of this voltage drop ito the total potential ditferencegives the value sought.

Furthermore, a diode tube may be inserted in the measuring circuit so as also to measure alternating voltages (voltage peaks).

Having thus described the invention, what is claimed is:

1. An electronic voltmeter comprising an electronic visual indicating tube of the tuning indicator tube type having at least a cathode, a grid and an anode, in combination with a first source of known voltage, a second source having an unknown voltage to be measured, each of said sources having poles of positive and negative polarity, a connection between the negative poles of said sources for maintaining said poles at the samepoten'tial, and two rheostats associated mechanically in such fashion that the sum of the :connect the grid'of said tube, at will, either with the positive pole of the source of unknown voltage, or through the second rheostat, with the negative pole of said sources.

2. An electronic voltmeter-according to claim 1 in which each of the two rheostats is composed of a series of ten resistances of one ohm each and a series of resistances of ten ohms each, and a movable connection means adapted to connect any resistance of the first series with any resistance of the secondseries.

3. An electronic voltmeter according to claim -1 in combination with means for changing the voltage impressed on the first voltage source and applied to the elements of the electronic tube.

4. An electronic voltmeter according to claim :1

in combination with :switchesadapted todisconnect the ;;negative pole :of the source of unknown vo1tage;as well as theends of the two rheostats Previously connected and to connect the other 9 ends of these opposed rheostats with the positive poles of the two voltage sources, the positive pole of the source of unknown voltage being then connected either to the grid of the electronic tube or the negative pole of source of known voltage through intermediary of a resistance.

5. An electronic voltmeter according to claim 1 in which the electronic tube is mounted in a movable insulating support, said support comprising a handle adapted to permit the operator to hold the same like a pistol, a contact prod at the front of said support, a switch mounted on said support, a trigger adapted to actuate said switch, the visual indicator portion of said electronic tube being directed rearwardly and the conductors which connect the elements of said tube to the voltage sources and to the rheostats of the voltmeter being grouped in a flexible cable, said switch being inserted in the grid circuit of the tube and adapted at will to connect said grid with said contact prod.

6. An electronic voltmeter comprising an electronic visual indicating tube of the so-called tuning indicator tube type provided at least with a cathode, a grid, an anode and a fluorescent screen giving a reading when bombarded by the electrons emitted by the cathode, means to apply between the grid and the cathode of said tuning indicator tube alternatively the voltages of a source of which the voltage is known and of a source of which the voltage is unknown and is to be measured, and means to vary the voltage of the source of which the voltage is known without changing the impedance of the circuit comprising said source and the tuning indicator tube, until the reading of said tuning indicator tube is maintained constant, the unknown voltage then being equal to the known voltage.

7. An electric voltmeter comprising an electronic visual indicating tube of the "tuning indicator tube type in combination with a first source of known voltage, a second source of which the unknown voltage is to be measured, circuits for connecting said sources to said tube, means for closing said circuits successively, a first rheostat for modifying the voltage of the first source acting on the tube, a second rheostat mechanically connected with the first rheostat in such a fashion that the impedance of the circuit of the first source remains constant with variation of the positions of said rheostats, one of the rheostats having graduations to permit reading the voltage of the second source when the tuning indicator tube gives the same indication under the action of the two sources.

MAURICE FOUQUART.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,171,216 Koch Aug. 29, 1939 2,175,690 Happe Oct. 10, 1939 2,235,173 Shepard Mar. 18, 1941 2,263,932 Schnoll Nov. 25, 1941 2,272,849 Perkins Feb. 10, 1942 2,359,572 McWhirter et a1 Oct. 3, 1944 

